This invention relates to a position control system wherein the position of a cage is determined by counting the number of digital pulses are generated with the movement of the cage.
A conventional system which determines the position of a cage by such counting operation is disclosed in Japanese Patent Application Laid-open NO. 59-53379 in which.
FIG. 6 illustrates a similar arrangement. In the figure, numeral 1 designates a control device for an elevator, and numeral 2 a driving device for driving a motor 3. A speedometer 4 to be used for a speed control is directly coupled to the shaft of the motor 3 and produces pulse signals proportional to the rotational frequency of the motor 3. Numeral 5 indicates a hoist driven by the motor 3, numeral 6 a deflecting sheave, symbol 6a a main rope, numeral 7 a counterweight, and numeral 8 a cage. Shown at numeral 9 is a fixed point detecting switch mounted on a hoistway wall 9a. When the cage 8 has just arrived at a floor F, the switch 9 is actuated by a cam 10 mounted on the cage 8 and provides a fixed point signal 9s. An endless steel tape 11 is extended between a tape wheel 12 which is disposed at the lowermost position of a hoistway, and a tape wheel 13 which is disposed at the uppermost position of the hoistway. It is fixed to the cage 8 midway, to rotate the tape wheels 12 and 13 in accordance with the movement of the cage 8. A rotary disc 14 is directly coupled to the rotary shaft 13a of the tape wheel 13, and is formed with slits 14a along its outer periphery. A pulse encoder 15 includes a light projector on one side and a light receptor on the other side with the rotary disc 14 intervening therebetween. It senses light passing through the slits 14a, to generate signal pulses and transmit them to the counter 16 of the control device 1 so as to detect the movement distance of the cage 8.
Next, the operation of the prior-art system will be described.
First, the cage 8 is caused to arrive at the floor F by a maintenance run. At this time, the fixed point detecting switch 9 comes in fit engagement with the cam 10 and sends the fixed point signal 9s to the control device 1. The counter 16 is set to an initial value by the fixed point signal 9s.
When an up run command is issued from the control device 1, the motor 3 brings the cage 8 to an up run. With the ascent of the cage 8, the steel tape 11 moves to rotate the rotary disc 14. The signal pulses are generated from the pulse encoder 15 by the slits 14a. These signal pulses are added to the initial value of the counter 16. Since the generation interval of the pulse signals corresponds to a distance, the position of the cage 8 can be detected on the basis of the number of the signal pulses.
Subsequently, when a down run command is issued, the motor 3 brings the cage 8 to a down run. In the down run, each time a signal pulse is generated from the pulse encoder 15, it is subtracted from the content of the counter 16.
In this manner, the count value of the counter 16 is subjected to addition or subtraction with the ascent or descent of the cage 8. Besides, the generation interval of the signal pulses corresponds to the distance. Therefore, the position of the cage 8 can be detected from the count value.
The prior-art position control system for the elevator, however, includes only one pulse encoder as described above. When the pulse encoder malfunctions, the position of the cage cannot be properly detected. In consequence, the door of the cage might open despite the fact that the cage has stopped between floors. In case of such a malfunction, passengers not only cannot go to desired floors but a very dangerous situation occurs in that the passengers possibly will not realize that the cage has stopped between floors and could fall from the cage into the hoistway.